Permanent magnet direct current motors are sometimes used for submerged fuel pump applications. These motors typically employ either face-type commutators or cylinder or "barrel"-type commutators. Face-type commutators have planar, circular commutating surfaces disposed in a plane perpendicular to the axis of armature rotation. Barrel-type commutators have arcuate, cylindrical commutating surfaces disposed on the outer side surface of a cylinder that is positioned coaxially around the axis of armature rotation. Regardless of their commutating surface configurations, electric motors used in submerged fuel pump applications must be small and compact, have a long life, be able to operate in a corrosive environment, be economical to manufacture and operate and be essentially maintenance-free.
Submerged fuel pump motors must sometimes operate in a fluid fuel medium containing an oxygen compound, such as methyl alcohol and ethyl alcohol. The alcohol increases the conductivity of the fuel and, therefore, the efficiency of an electrochemical reaction that deplates any copper motor components that are exposed to the fuel. For this reason, carbon and carbon compositions are sometimes used to form carbon segments with segmented commutating surfaces for the motors. This is because carbon commutators do not corrode or "deplate", as copper commutators do. Commutators with carbon segments also typically include metallic contact sections that are in electrical contact with the carbon segments and provide a terminal for physically connecting each electrical contact to an armature coil wire.
It is known to form a carbon commutator by first molding and heat treating a moldable carbon compound or machining heat-treated carbon or carbon/graphite stock. Such an arrangement is shown in German Disclosure 3150505.8. A commutator-insulating hub may then be formed to support the metallic substrate. The hub may be molded directly to the metallic substrate either before or after the carbon is bonded to the metallic substrate. Slots are then machined through the carbon article and the metallic substrate to separate the carbon article and substrate into a number of electrically isolated segments. An inner diameter, outer diameter and the commutating surface of the commutator may also need to be machined.
After the completed commutator is assembled to an armature, a clamshell mold may be positioned over the newly assembled commutator-armature in a final overmolding process. With face-type commutators, an open end of the clam shell mold is made to seal around the commutator in a manner that leaves the commutating surface exposed. Insulator material is then injected into the clam shell mold. Once the insulator material has cured, the clam shell mold is removed. This final overmolding step protects copper armature windings and other corrosion-prone elements from chemically reacting with ambient fluids such as oxygenated fuels. The overmolding also secures wires to reduce potential for stress failures and to maintain a corrected dynamic balance level. Overmolding will also reduce windage losses in the pump.
When, in manufacturing a carbon commutator with a metallic substrate, cuts are machined into or through the metallic substrate, metal chips may be produced. These metal chips can lodge in the slots between carbon segments causing electrical failures. Machining into a metallic substrate can also expose the cut portions of the substrate to the corrosive effects of oxygenated fuels.
Where the carbon and metal substrate portions of a commutator are machined-through to form electrically isolated segments, some type of support structure must be provided to strengthen the commutator and mechanically bind the carbon segments and conductor sections together. Such support structures sometimes require substantial additional axial space for the commutator, which can increase the overall axial length of the armature-commutator assembly and or reduce the size and the quantity of wire wound in the armature.
For some types of electrical-conducting resin-bonded carbon compositions, an insulating surface skin characteristically forms on exterior surfaces of the composition as it cures. This skin forms an impediment to electrical contact between the carbon composition and the metallic conductor sections. Therefore, a carbon commutator using such a composition must provide an electrical path through the insulating surface skin.
One approach to solving these problems is disclosed in U.S. Pat. No. 5,386,167 issued Jan. 31, 1995 to Strobi (the Strobi patent). The Strobi patent shows a face-type commutator having eight carbon segments formed from an electrical-conducting resin-bonded carbon composition. To avoid problems associated with machining into metal substrates, the carbon segments are formed by overmolding a carbon disk onto eight pie-piece-shaped copper segments then radially cutting between the segments to form the electrically isolated carbon segments. A plastic substrate holds the copper segments in position for carbon overmolding and provides mechanical interlock between the carbon segments. However, the plastic substrate increases the axial thickness of the commutator. In addition, the Strobi patent does not provide structures that would provide an electrical path through carbon composition skinning or structures that might otherwise reduce electrical resistance.
U.S. Pat. No. 4,358,319 issued Nov. 9, 1982 to Yoshida et al. discloses a barrel-type carbon commutator assembly that includes an annular cylindrical array of carbon segments. Each carbon segment has an outer semi-circumferential side surface for making physical and electrical contact with a brush. A retention groove extends around an inner circumferential surface of the carbon segment array. The carbon segments are electrically isolated from each other by longitudinal cuts. A hub comprising insulating material is disposed within the annular carbon segment array and engages the retention groove at the top end of each carbon segment.
To manufacture this commutator Yoshida et al. discloses a method that includes the steps of forming an annular carbon cylinder with a retention groove, over-molding the carbon cylinder with insulator material to form a hub and machining slots in the over-molded barrel to form electrically isolated barrel segments. The electrical connections between carbon segments and coil wires are made by soldering or gluing the wires directly to the carbon segments themselves.
A fuel pump supplied by Bosch to Mercedes Benz shows a barrel-style commutator that includes a cylindrical commutating surface formed by a cylindrical array of carbon segments. Radial inner surfaces of the carbon segments form a composite inner circumferential surface of the carbon segment array. The carbon segments are electrically connected to respective coil wires by copper substrate sections soldered to the respective radial inner surfaces of the carbon segments. Each copper substrate section includes a terminal for supporting the end of a coil wire.
The Bosch commutator appears to be formed by fitting and soldering a tube portion of a copper substrate to the inner circumferential surface of the carbon cylinder. Radial cuts are then made to form and electrically isolate the carbon segments and copper substrate sections from each other. An over-molded insulator holds the carbon segments and copper substrate sections together. This process requires that a copper substrate be fabricated to include wire terminals and a tube portion closely toleranced to fit within the inner circumferential surface of the carbon cylinder. The Bosch process also requires that a difficult soldering operation be performed between the inner circumferential surface of the carbon cylinder and the outside diameter of the copper tube.
U.S. Pat. No. 5,255,426 issued Oct. 26, 1993 to Farago et al. discloses a face-type carbon commutator manufactured by first forming an annular or torroidal carbon cylinder comprising fine-grained electrical-grade carbon. Next, a cylinder base end surface is plated with a layer of nickel. A layer of copper is then plated over the nickel plating. The plated base end surface of the cylinder is then soldered to a stamped and formed copper substrate mounted on a pre-molded hub. Lateral slots are then machined axially downward into a top commutating surface opposite the base surface of the carbon cylinder. The slots are cut axially through the carbon and the copper substrate to form the electrically isolated carbon/copper commutator sectors. After the slots are machined, the pre-molded hub continues to hold the electrically isolated commutator sectors together.
What are needed are both face and barrel-type carbon-segment commutators that are stronger and provide lower electrical resistance through improved electrical contact between carbon segments and metallic substrates. Also needed are methods for manufacturing such commutators that are quick, easy and inexpensive.